keeping the AGND plane seperate
The separate analog ground plane is something you don't particularly need to worry about until you have additional analog components in your design. If the analog inputs of your AVR were attached to some analog amplifiers and external analog muxes and stuff like that, you'd want those components and the analog pins of the AVR to share a ground plane that was isolated from your digital circuitry. Since the average Arduino analog connection is some sort of questionable jumper wired plugged into a flakey socket leading to a surplus component on a breadboard, getting too paranoid is unlikely to be needed. You have to keep in mind the overall accuracy and goodness of your design; I laugh inside when people whine about needing more than a 10bit A2D, but are using 1% resistors... (there are times when this is NOT silly, but still...)